Wide frequency range delay locked loop

ABSTRACT

A delay locked loop operates over a wide range of frequencies and has high accuracy, small silicon area usage, low power consumption and a short lock time. The DLL combines an analog domain and a digital domain. The digital domain is responsible for initial lock and operational point stability and is frozen after the lock is reached. The analog domain is responsible for normal operation after lock is reached and provides high accuracy using smaller silicon area and low power.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/523,406, filed Jun. 14, 2012, which is a continuation of U.S.application Ser. No. 13/186,104, filed Jul. 19, 2011, now U.S. Pat. No.8,213,561, issued Jul. 3, 2012, which is a continuation of U.S.application Ser. No. 11/999,162, filed Dec. 4, 2007, now U.S. Pat. No.8,000,430, issued Aug. 16, 2011, which is a continuation of U.S.application Ser. No. 10/335,535, filed Dec. 31, 2002, now U.S. Pat. No.7,336,752, issued Feb. 26, 2008.

The entire teachings of the above application(s) are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

Many devices such as synchronous dynamic random access memory (SDRAM)and microprocessors receive an external clock signal generated by anexternal clock source such as a crystal oscillator. The external clocksignal received through an input pad on the device is routed to variouscircuits within the device through a tree of buffer circuits. The buffertree introduces a common delay between the external clock and eachbuffered clock.

Typically, a delay locked loop (DLL) with an adjustable delay line isused to synchronize the buffered clock signal with the external clocksignal by delaying the external clock signal applied to the buffer tree.The DLL includes a phase detector, which detects the phase differencebetween the external clock signal and a buffered clock signal. Based onthe detected phase difference, the DLL synchronizes the buffered clocksignal to the external clock signal by adding an appropriate delay tothe external clock signal until the buffered external clock signal (theinternal clock) is in phase with the external clock signal. The DLL canbe implemented as an analog delay locked loop or a digital delay lockedloop. In an analog delay locked loop, a voltage controlled delay line isused to delay the external clock signal.

FIG. 1 is a block diagram of a prior art analog delay locked loop (DLL)100. The analog DLL 100 synchronizes an internal clock signal CK_(I)with an external clock signal CK_(E). The external clock CK_(E) signalis coupled to a voltage controlled delay line 102, and the voltagecontrolled delay line 102 is coupled to clock tree buffers 108. Thedelayed external clock signal CK_(E) is fed into the clock tree buffers108 where it propagates to the outputs of the tree and is applied to thevarious circuits. The delay through the clock tree buffer 108 results ina phase difference between the external clock signal CK_(E) and theinternal clock signal CK_(I). The voltage controlled delay line 102 addsfurther delay to the external clock signal CK_(E) to synchronize theexternal and internal clock signals.

To determine the appropriate delay in the delay line, one of the outputsof the clock tree buffers 108 is coupled to a phase detector 104 whereit is compared with the external clock signal CK_(E). The phase detector104 detects the phase difference between the internal clock CK_(I) andthe external clock CK_(E). The output of the phase detector 104 isintegrated by a charge pump 106 and a loop filter capacitor 112 toprovide a variable bias voltage V_(CTRL) 110 for the voltage controlleddelay line (VCDL) 102. The bias voltage V_(CTRL) selects the delay to beadded to the external clock signal by the VCDL 102 to synchronize theinternal clock signal CK_(I) with the external clock signal CK_(E).

The phase detector 104 can be a D-type flip-flop with the D-inputcoupled to the external clock signal CK_(E) and the clock input coupledto the internal clock signal CK_(I). On each rising edge of the internalclock signal CK_(I), the output of the phase detector 104 indicateswhether the rising edge of the internal clock signal is before or afterthe rising edge of the external clock signal.

The analog DLL 100 produces a voltage controlled delay with highaccuracy. However performance of the analog DLL varies over a frequencyrange because of a non-linear control voltage characteristic.

FIG. 2 is a graph illustrating the non-linear control voltagecharacteristic for the voltage controlled delay line shown in FIG. 1. Ingeneral, devices support a wide range of external clock frequencieswithin which an operational frequency is selected for a particulardevice. In the example shown in FIG. 2, the device can operate at anyfrequency between point A and point C. The operational frequencyselected is at point B.

As shown, the control voltage characteristic is non-linear: sharp at oneend of the control voltage range (point C) and almost flat at theopposite end (point A). This control voltage characteristic results inDLL instability at point C and long lock times at point A. The widerange of frequencies (delays) is controlled by the bias voltageV_(CTRL).

Referring to FIG. 1, the bias voltage V_(CTRL) is the output of thecharge pump 106, which remains in a high-impedance state most of thetime. Any noise on the bias voltage signal V_(cTL) disturbs the outputof the analog DLL 100. For example, if the analog DLL is operating atpoint B, a small voltage change (ΔV) due to noise results in a largechange in delay. Thus, the analog DLL is very sensitive to noise whenoperating at point B, within the wide frequency range shown from point Cto point A. Therefore, the analog DLL is not stable within a widefrequency range.

A digital DLL does not have the stability problem of an analog DLL.However, the accuracy of a digital DLL is not the same as the accuracyof an analog DLL, because the delay is provided by combining fixedquantum (steps) of delay. The smaller the step of delay, the higher theaccuracy. However, a decrease in step size results in a correspondingincrease in silicon area because more delay elements are required tocover the wide frequency range.

SUMMARY OF THE INVENTION

A delay locked loop, which has high accuracy, good stability and a fastlock time over a wide frequency range is presented. The delay lockedloop combines shorter lock time, good accuracy and stability with lowpower consumption and small silicon area for the delay locked loopoperating in a wide range of frequencies.

The delay locked loop includes a digital delay circuit and an analogdelay circuit. The digital delay circuit engages delay elements toprovide coarse phase adjustment in the delay locked loop. The analogdelay circuit provides a fine phase adjustment in the delay locked loopwhile the digital delay circuit is held at a fixed delay. A lockdetector in the digital delay circuit detects completion of the coarsephase adjustment, freezes the fixed delay upon completion and enablesfine phase adjustment.

The digital delay circuit, which includes a plurality of fixed delayelements, operates in a wide delay range. The analog delay circuitoperates in a small delay range within the wide delay range and is heldat a second fixed delay until the digital delay circuit completes thecoarse phase adjustment.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a block diagram of a prior art analog delay locked loop (DLL);

FIG. 2 is a graph illustrating the non-linear controlling voltagecharacteristic for the voltage controlled delay line shown in FIG. 1;

FIG. 3 is a block diagram of a wide frequency range delay locked loopaccording to the principles of the present invention;

FIGS. 4, 4A and 4B illustrate delay cells in the DCDL and the VCDL;

FIG. 5 is a schematic of one embodiment of any one of the delay cellsshown in FIG. 4;

FIG. 6 is a graph illustrating the non-linear controlling voltagecharacteristic for the narrow frequency range of the VCDL in the DLLshown in FIG. 3;

FIG. 7 is a schematic of an embodiment of the lock detector and theanalog switch shown in FIG. 3;

FIGS. 8A-C are timing diagrams illustrating the relationship of thephase detector output to the phase difference between the clocks; and

FIG. 9 is a timing diagram illustrating signals in the schematic shownin FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

While this invention has been particularly shown and described withreferences to example embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

A description of preferred embodiments of the invention follows.

FIG. 3 is a block diagram of a wide frequency range delay locked loop(DLL) 300 according to the principles of the present invention. The widefrequency range DLL 300 has two domains of operation: a digital domainwhich includes a digital delay circuit 302 and an analog domain whichincludes an analog delay circuit 304.

In a DLL, high accuracy, small silicon area usage and lower power aretypically achieved using an analog technique, while good stability andshorter lock times are typically achieved with a digital technique. Thewide frequency range DLL 300 combines the two techniques to provide highaccuracy, good stability and a fast lock time over a wide frequencyrange. The digital delay circuit 302 is responsible for coarse phaseadjustment during initialization and the analog delay circuit 304 isresponsible for fine phase adjustment during normal operation, aftercoarse phase adjustment is completed by the digital delay circuit 302.The digital delay circuit 302 operates within the wide delay range andbrings the delay locked loop 300 to a stable operation point duringpower-up initialization. In normal operation, the analog delay circuit304 operates within a small delay range of the stable operation pointwithin the wide delay range and maintains the delay locked loop at thestable operation point while the digital delay circuit 302 is held at afixed delay.

The overall delay provided by the DLL includes a digitally controlleddelay line (DCDL) 306 having a set of delay elements, each having afixed delay and a voltage controlled delay line (VCDL) 312. Thecombination of the DCDL delay provided by the DCDL 306 and the VCDLdelay provided by the VCDL 312 provides an accurate delay. Only one ofthe domains can vary the DLL delay at any time. At power-upinitialization, the digital delay circuit 302 varies the DCDL 306(coarse delay). After coarse phase adjustment is complete (lock isreached), the DCDL delay is held at a fixed number of DCDL delayelements (frozen) and the analog delay circuit 304 varies the DLL delayto provide fine phase adjustment by varying the VCDL delay.

The digital delay circuit 302 operates within the wide delay range tobring the DLL 300 to the operation point (lock) quickly to provide ashort lock time. A lock detector 310 in the digital delay circuit 302detects when the digital delay circuit 302 has brought the DLL delay tothe stable operation point and enables control of the DLL delay to beswitched to the analog delay circuit 304.

A phase detector 320 detects the phase difference between the externalclock signal CK_(E) and the internal clock signal CK_(I). The phasedetector 320, can be any phase detector well known to those skilled inthe art. In the embodiment shown, the phase detector 320 (FIG. 3)includes a D-type flip-flop with CK_(I) connected to the clock input andCK_(E) connected to the D-input. The rising edge of CK_(I) latches thestate of CK_(E) at the output (Ph_det) of the D-type flip-flop.

The analog delay circuit 304 includes a multiplexor 314, a VCDL 312 anda charge pump 316. The VCDL 312 is a chain ofdifferential-input-differential-output stages (delay elements) withvoltage control. The multiplexor 314 selects the source of the VCDL biasvoltage 322 to the VCDL 312. The VCDL bias voltage 322 is a fixed biasvoltage V_(BPI), V_(BNI) provided by bias voltage generator 318 or avariable bias voltage V_(BP2), V_(BN2) provided by charge pump 316.During initialization, before the DCDL 306 achieves lock, differentialbias voltage V_(BPI), V_(BNI) provides the VCDL bias voltage 322 throughmultiplexor 314. Thus, while the digital delay circuit 302 selects theDCDL delay, the VCDL bias voltage 322 provides a constant VCDL delay.That delay may be in the middle of the full delay range of the VCDL toenable fine tuning in both positive and negative directions as discussedbelow.

At initialization, the code stored in a counter 308 is initialized tozero, which corresponds to the minimum delay; that is, the minimumnumber of delay cells in the DCDL 306 that are engaged. The lockdetector 310 allows the DCDL 306 to increase the DCDL delay by addingdelay cells as the counter 318 is incremented until the nearest risingedge of the internal clock signal CK_(I) is aligned with the externalclock signal CK_(E). The counter 308 is incremented by the externalclock signal CK_(E) until lock is reached (the clocks are aligned). Inone embodiment, the counter 308 is an up counter which increments oneach rising edge of the external clock signal CK_(E) while enabled bythe SW signal from the lock detector 310. Delay cells in the DCDL 306are added to the DCDL delay line based on the n-bit count value outputby the counter 308 to engage the minimum number of DCDL delay cellsnecessary dependent on the bias voltage V_(BPI), V_(BNI).

After the clocks are aligned, the SW signal output by the lock detector310 disables any further incrementing of the counter 308. The VCDL biasvoltage 322 is provided by bias voltage V_(BP2), V_(BN2), the output ofcharge pump 316, through multiplexor 314. The charge pump 316 can be anycharge pump well known to those skilled in the art.

By engaging only the minimum number of delay cells in the DCDL 306, theoverall delay line is minimum length to minimize noise. Once lock isreached, the digital delay circuit 302 is held at a fixed delay bydisabling further incrementing of the counter 308. Only the VCDL portionof the DLL delay line can be varied by the analog delay circuit 304. Theanalog delay circuit 304 fine tunes the DLL delay to compensate for alldrifts and condition changes to keep the external and internal clocksignal edges aligned, by varying the VCDL delay, which is added to thefixed delay provided by the DCDL. The analog controlled delay line 310varies the VCDL delay up or down by varying the bias voltage to the VCDLdelay cells 402 based on detected phase difference between the clocks.

FIG. 4 illustrates delay cells in the DCDL and the VCDL. The digitallycontrolled delay line (DCDL) includes a chain of DCDL delay cells 400and the voltage controlled delay line (VCDL) includes a chain of VCDLdelay cells 402. The delay of each DCDL cell 400 is fixed by permanentlyconnecting the bias voltage for each DCDL cell 400 to a fixed biasvoltage V_(BP1), V_(BN1). The fixed bias voltage V_(BP1), V_(BN1) isprovided by a bias voltage generator 318 (FIG. 3) which can be any typeof voltage stabilizing device, for example, a band-gap reference andneed not correspond to the VCDL bias voltage 322 initially applied tothe VCDL.

At initialization, none of the delay elements 400 in the DCDL 306 areengaged. The DLL delay includes only the fixed delay provided bydemultiplexor 404, multiplexor 408 and the VCDL delay elements 402 inthe VCDL connected to the fixed bias voltage V_(BP1), V_(BN1). The VCDLdelay provided by VCDL is dependent on the fixed bias voltage V_(BP1),V_(BN1). In the embodiment shown, the DCDL delay cells 400 and the VCDLdelay cells 402 are the same delay cell with voltage controlled delay.However, in an alternate embodiment, the DCDL delay cell 400 can differfrom the VCDL delay cell 402.

The DCDL is initially variable by increasing the number of DCDL delayelements 400 with each DCDL delay element 400 having the same delayfixed by the fixed bias voltage V_(BP1), V_(BN1). In the embodimentshown, during initialization the same fixed bias voltage V_(BP1),V_(BN1) is coupled to the DCDL delay elements 400 and the VCDL elements402. However, in alternate embodiments, the fixed bias voltage coupledto the VCDL delay elements 402 and the DCDL delay elements 400 can bedifferent; for example, a first bias voltage may be set to 0.7V_(DD)coupled to the DCDL and a second bias voltage may be set to 0.5V_(DD)coupled to the VCDL. The VCDL delay is initially fixed with each of thethree VCDL delay elements 402 numbered 1-3 coupled to the fixed biasvoltage V_(BP1), V_(BN1), but the VCDL delay varies with changes in theVCDL bias voltage 322 during normal operation.

The number of engaged elements in the DCDL 306 is dependent on the n-bitcount 406 output by the counter 308. The n-bit count 406 is coupled tothe de-multiplexor 404 to select the output of the de-multiplexor 404through which the external clock is output to the DCDL 306. The n-bitcount 406 is also coupled to multiplexor select logic 430 which providesan m-bit multiplexor select signal, with one of the m-bits coupled toeach multiplexor in the DCDL 306. In one embodiment the multiplexorselect logic 430 is a decoder which decodes the n-bit count to providethe m-bit multiplexor select signal. In the embodiment shown m is 7 andn is 3. There are with six delay elements 400 labeled 4-9. Themultiplexor select logic 430 decodes a three bit count 406 to select oneof the seven multiplexors through which to forward the external clock asshown in Table 1 below. The Most Significant Bit (MSB) of the seven bitmultiplexor select signal corresponds to the select signal formultiplexor 420 and the Least Significant Bit (LSB) of the seven bitmultiplexor signal corresponds to the select signal for multiplexor 408.Thus, as the count increases the number of delay elements engagedincreases. In an alternate embodiment, the multiplexor select logic canbe implemented as a shift register clocked by the external clock andenabled by the SW signal.

TABLE 1 Count Multiplexor select De-multiplexor select count [2:0]mux_en [6:0] demux_sel [6:0] 000 1111110 1111110 001 1111101 1111101 0101111011 1111011 011 1110111 1110111 100 1101111 1101111 101 10111111011111 110 0111111 0111111

After lock has been reached, the external clock signal CK_(E) is delayedthrough DCDL delay elements engaged dependent on the n-bit count outputby counter 308. Control of the DLL delay is switched to the VCDL 312 byswitching the bias voltage V_(BP1), V_(BN1) to the bias voltage V_(BP2),V_(BN2) through the multiplexor 314 (FIG. 3).

Thus, the DLL delay includes minimum delay provided by the engaged DCDLdelay elements 400 in the DCDL 306 and additional delay provided by theVCDL 312 to provide an accurate DLL delay. The stability of the DLL isincreased by using the digital domain to cover a wide delay range toobtain a minimum delay, then freezing the digital domain to allow theanalog domain to operate within a small delay range to control the DLLdelay. The bias voltage coupled to the VCDL bias voltage 322 is set sothat the VCDL does not control the DLL delay until after lock isdetected by the digital domain. Before lock, the VCDL merely provides aconstant delay independent of the phase difference between the clocks.

Initially the counter 308 is reset to 0. The de-multiplexor 404 directsthe external clock CLK_(E) to engage delay elements dependent on then-bit count 406 output by the counter 308. With count 406 set to ‘0’,CLK_(E) is directed through output 422 of de-multiplexor 404 coupled tomultiplexor 408 and no delay DCDL elements 400 are engaged.

After the counter 308 is incremented to ‘1’ by CLK_(E), CLK_(E) isdirected through output 424 of the de-multiplexor 404 by count 406 setto ‘1’ to engage DCDL delay stage labeled 4. Multiplexor 410 is enabledto allow CLK_(E) through to DCDL delay stage 400 labeled 4 and the m-bitmultiplexor select signal output by multiplexor select logic 430 allowsdelayed CLK_(E) through multiplexor 408 to the VCDL.

All six DCDL delay stages are engaged when the count 406 is six, andCLK_(E) is directed through de-multiplexor output 426 throughmultiplexors 420, 418, 416, 414, 412, 410, 408 and delay elementslabeled 9-4. The DCDL line is frozen when the counter 308 is disabled bythe SW signal.

FIG. 5 is a schematic of one embodiment of any one of the delay elementsshown in FIG. 4. The delay cell 400 includes a source-coupled pair ofNMOS devices T1, T2 with symmetric loads 500, 502.

The differential input clock signal CLK_(E)I−, CLK_(E)I+, is coupled tothe respective gates of NMOS devices T1, T2 with CLK_(E)I+ coupled tothe gate of NMOS device T1 and CLK_(E)I− coupled to the gate of NMOSdevice T2. The differential output clock signal CLK_(E)O−, CLK_(E)O+, iscoupled to the respective drains of NMOS devices T1, T2. The sources ofNMOS devices T1 and T2 are coupled and are also coupled to the drain ofNMOS current source T3. NMOS current source T3 compensates for drain andsubstrate voltage variations.

Symmetric load 500 includes a diode-connected PMOS device T4 connectedin parallel with a biased PMOS device T5. Symmetric load 502 includes adiode-connected PMOS device T7 connected in parallel with a biased PMOSdevice T6. The effective resistance of the symmetric loads 500, 502changes with changes in the bias voltage V_(BP1) resulting in acorresponding change in delay through the delay stage from thedifferential clock input to the differential clock output.

FIG. 6 is a graph illustrating the non-linear control voltagecharacteristic for the narrow delay range of the VCDL 312 in the DLL 300shown in FIG. 3. In the embodiment shown, the digital domain providesthe minimum delay to bring the operating range of the DLL 300 to pointB. After lock, the analog domain operates within a narrow delay range600 from point B-High to point B-Low. This delay range is much smallerthan the wide delay range supported by the DLL, but may be controlled bythe same large voltage range as applied in the pure analog case of FIG.2. The small delay range controlled by a large voltage range ensures thestability of the analog domain during normal operation of the DLL.

As shown, the analog delay circuit 304 operates within the delay range85 ns to 80 ns over voltage range 200 mV to 800 mV. In contrast to thewide delay range over the same voltage range shown in FIG. 2, a smallvariation in control voltage (ΔV) does not substantially affect thedelay.

FIG. 7 is a schematic of an embodiment of the lock detector 310 and themultiplexor 314 shown in FIG. 3. The lock detector 310 includes two SRflip-flops 700, 702, AND gate 706 and inverter 704. SR flip-flop 700detects when the internal clock signal CK_(I) is within a phasedetection window. SR flip-flop 702 detects when the internal clocksignal CK_(I) is in phase with the external clock signal CK_(E). Oncethe internal clock signal CK_(I) is in phase with the external clocksignal CK_(E) the SW signal is set to logic ‘0’ to disable furtherchanges to the DCDL delay.

The lock detector output SW is set to logic ‘0’ prior to lock beingreached and set to logic ‘1’ after lock is reached. Prior to lock beingreached, the logic ‘0’ on the SW signal couples the fixed bias voltagethrough multiplexor 314 to provide the VCDL bias voltage 322. After lockhas been reached, the logic ‘1’ on SW couples the variable bias voltageV_(BPN2), V_(BPN2) through multiplexor 314 to provide the VCDL biasvoltage 322, to allow the VCDL 312 to fine tune the overall delay.

On power up, the reset signal coupled to the R-input of the SR flip-flop700 and the SR flip-flop 702 is set to logic ‘1’. Both flip-flops 700,702 are reset with the respective Q outputs (LC1, SW) set to logic ‘0’.The SR flip-flops 700, 702 remain in a reset state with logic ‘0’ on therespective Q outputs until the phase detector 320 detects that the phasedifference between clock signals CK_(E), CK_(I) are in the phasedetection window. The phase difference is within the phase detectionwindow while the rising edge of the internal clock signal CK_(I) isafter the falling edge of the external clock signal CK_(E). The outputof the phase detector (Ph_det) changes to logic ‘0’. The logic ‘0’ onPh_det changes the S-input of SR flip flop 700 to logic ‘1’ throughinverter 704 which sets SR flip-flop 700 (i.e. the Q output changes tologic ‘1’). The delay provided by the DCDL 306 continues to increasefurther delaying the rising edge of the internal clock signal until theinternal clock signal and the external clock signals are in phase. SRflip-flop 702 is set on the next rising edge of Ph-det which occurs whenthe rising edge of CK_(E) is detected after the rising edge of CK₁. TheQ output of SR flip-flop 702 is set to logic ‘1’. The logic ‘1’ on theoutput of SR flip-flop 702, the SW signal, disconnects the VCDL biassignal 322 from bias voltage V_(BP1), V_(BN1) through multiplexor 314and connects the bias signal V_(BP2), V_(BN2) from charge pump 316 (FIG.3) to the VCDL bias signal 322 to the VCDL 312.

The lock detector 310 remains in a locked state with SW set to logic ‘1’until the system is reset. While in the locked state, the digital domainno longer controls the delay because, while SW is set to logic ‘1’, thecode stored in the counter 308 is frozen to freeze the DCDL delay.

FIGS. 8A-C are timing diagrams illustrating the relationship of thephase detector output (Ph-det) to the phase difference between theclocks. Referring to FIG. 8A, at initialization, the phase detector 320(FIG. 3) detects that the internal clock rising edge is after theexternal clock rising edge. The rising edge of CK₁ latches a ‘1’ on thePh_det output of the D-type flip-flop. The CK_(E) rising edge continuesto increment the code to add additional delay to the DCDL.

Referring to FIG. 8B, the phase detector detects that the CK_(I) risingedge is now after the falling edge of CK_(E). The rising edge of CK_(I)latches a ‘0’ on the Ph_det output of the D-type flip-flop. The CK_(E)rising edge increments the code to add further delay cells to the DCDL.

Referring to FIG. 8C, the phase detector detects the lock condition whenthe CK_(I) rising edge moves after the CK_(E) rising edge. The risingedge of CK_(I) latches a ‘1’ on the Ph_det output of the D-typeflip-flop.

FIG. 9 is a timing diagram illustrating signals in the schematic shownin FIG. 7. The timing diagram shows the state of signals in theschematic when the system is reset, upon detecting that the phasedetection window has been reached and upon detecting lock. FIG. 9 isdescribed in conjunction with FIG. 3 and FIG. 7.

At time 900, the system is reset and the reset signal set to logic ‘1’.The reset signal is coupled to the R-inputs of flip-flops 700, 702 toreset the flip-flops. The Ph_det signal is reset to logic ‘1’. The Qoutputs (LC1, SW) of both flip-flops are reset to ‘0’. The internalclock signal CK₁ has the same frequency as the external clock signalCK_(E) but there is an initial phase difference due to the delay ofCK_(E) through the clock tree buffers 328.

At time 802, after the system is reset, the reset signal changes tologic ‘0’. Initially delay is added to CK_(E) through the VCDL and nodelay is added through the DCDL. The rising edge of CK₁ occurs laterthan the rising edge of CK_(E) due to the delay through the clock treebuffers 328 (FIG. 3) and the delay through the VCDL. The SW signal setto logic ‘0’ allows CK_(E) to increment the code stored in the counter308 (FIG. 3). As the code stored in the counter 308 (FIG. 3) isincremented by CK_(E) (rising edge or falling edge), more delay elements400 (FIG. 4) are added to the DCDL 306 (FIG. 3) to further delay CK_(E).The delay between CK_(E) and CK₁ increases until the phase detectionwindow is reached.

At time 904, the phase detector 320 (FIG. 3) detects that the phasedetection window has been reached. The Ph_det signal output from thephase detector changes state from logic ‘1’ to logic ‘0’ indicating thatthe phase detector 320 has detected a rising edge of CK₁ signal after afalling edge of CK_(E). SR flip-flop 600 is set, and LC1 at the Q outputis set to ‘1’. In successive clock periods, the phase difference betweenCk_(E) and Ck_(I) decreases as the DCDL delay is increased.

At time 906, the phase detector 320 (FIG. 3) detects that the minimumDCDL delay has been added by the DCDL; that is the rising edge of CK_(I)occurred after the rising edge of CK_(E). The Ph-det output of the phasedetector 320 changes to logic ‘1’. LC2 at the output of AND gate 706changes to logic ‘1’, the SR flip-flop 702 is set and the Q output (SW)changes to logic ‘1’. Further changes on the Ph-det signal do not affectthe state of LC1 and SW. The SW signal set to ‘1’ disables furtherincrementing of the counter 308.

During normal DLL operation, the delay adjustment of the clock path tocompensate for drifts and condition changes covers a narrow range of thewide delay range. Thus, after the lock has been reached, the DCDLprovides the minimum delay. The DLL delay is varied by the VCDL inside asmaller delay range. Monitoring the smaller delay range during normaloperation provides more stability and reduces the controlling voltagenode sensitivity.

The invention has been described for an embodiment having a single fixedbias voltage level. In an alternate embodiment, more than one fixed biasvoltage level can be used to provide a more compact DLL that is lessnoise sensitive. This allows the wide delay range to be modified inorder to reduce the number of DCDL delay elements by selecting a fixedbias voltage level dependent on the frequency of the external clock.Reducing the number of DCDL delay elements, reduces sensitivity tonoise. For example, in one embodiment, with a fixed bias voltage of0.6V_(DD), fifteen DCDL delay elements are required to provide the DCDLdelay. When the fixed bias voltage is 0.7V_(DD), only eight DCDL delayelements are required to provide the DCDL delay. However, changing thedelay range may result in the delay range covering an unstable region,for example, at point C in the graph shown in FIG. 2.

The invention can be used in integrated circuits requiring high accuracyof input/output data synchronization, for example, in memory integratedcircuits.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims. For example, while the delay of theDCDL remains fixed over short times, it may be allowed to occasionallyshift as, for example, the VCDL approaches its delay limits.

What is claimed is:
 1. A memory device comprising: a delay locked loop;and an input for receiving an external clock signal for routing to thedelay locked loop, the delay locked loop being configured to synchronizean internal clock signal with the external clock signal, the delaylocked loop comprising: a digital delay circuit configured to enabledigital delay elements of the digital delay circuit in providing coarsephase adjustment in the delay locked loop, the digital delay circuitproviding a coarse delayed differential clock signal; an analog delaycircuit configured to provide fine phase adjustment in the delay lockedloop in response to a control signal, the analog delay circuit receivingthe coarse delayed differential clock signal and producing a finedelayed clock signal, the analog delay circuit comprising parallelsymmetric loads to cause the fine delay, the control signal beingoperative to vary an effective resistance of the symmetric loads;circuitry to provide the fine delayed clock signal as the internal clocksignal; a phase detector circuit configured to compare the externalclock signal and the internal clock signal; and a lock detector circuitin communication with the phase detector circuit and the analog delaycircuit, the lock detector circuit being configured to: i) hold thedigital delay circuit at a fixed delay; and ii) provide the controlsignal to the analog delay circuit.
 2. The memory device of claim 1,further comprising circuitry for providing the fine delayed clock signalto an output data synchronization circuit.
 3. The memory device of claim1, further comprising circuitry for causing a shift of the digital delaycircuit when the analog delay circuit approaches a delay limit.
 4. Thememory device of claim 1, further comprising circuitry for selecting oneof a plurality of fixed bias voltage levels depending on a frequency ofthe external clock signal.
 5. The memory device of claim 1 wherein thelock detector circuit is a part of the digital delay circuit.
 6. Thememory device of claim 1 wherein the analog delay circuit comprises ananalog delay line circuit.
 7. The memory device of claim 1 wherein thedigital delay elements are fixed digital delay elements.
 8. The memorydevice of claim 1 wherein the analog delay circuit is distinct from andin series with the digital delay circuit.
 9. The memory device of claim1 wherein the analog delay circuit is configured to be held at a secondfixed delay before the digital delay circuit completes the coarse phaseadjustment.
 10. The memory device of claim 1 wherein the analog delaycircuit includes a voltage controlled delay line circuit, a multiplexer,and a charge pump.
 11. The memory device of claim 1 wherein the lockdetector circuit includes at least two flip flops and an inverter. 12.The memory device of claim 1 wherein the memory device is a synchronousdynamic random access memory device.
 13. The memory device of claim 1wherein the digital delay elements form part of a digitally controlleddelay line, and the digitally controlled delay line further includes ademultiplexer.
 14. The memory device of claim 13 wherein the digitallycontrolled delay line further includes a plurality of multiplexersuniformly interspaced between the digital delay elements.
 15. The memorydevice of claim 14 wherein the digital delay elements are fixed digitaldelay elements.
 16. A method for performing phase adjustment in a delaylocked loop comprising: enabling digital delay elements of a digitaldelay circuit in providing coarse phase adjustment in the delay lockedloop to provide a coarse delayed differential clock signal; receivingthe coarse delayed differential clock signal at an analog delay circuitof the delay locked loop; applying a fine delay to the coarse delayeddifferential clock signal to output a fine delayed clock signal usingparallel symmetric loads of the analog delay circuit; using a controlsignal to vary an effective resistance of the parallel symmetric loads;in a phase detector circuit of the delay locked loop, comparing anexternal clock signal and the fine delayed clock signal; in a lockdetector circuit of the delay locked loop: holding the digital delaycircuit at a fixed delay and providing the control signal to the analogdelay circuit.
 17. The method of claim 16 wherein the digital delayelements are fixed digital delay elements.
 18. The method of claim 16,further comprising: when a delay limit of the analog delay circuit isapproached during the fine phase adjustment, enabling or disabling anadditional digital delay element of the digital delay circuit.
 19. Themethod of claim 16 wherein the analog delay circuit is distinct from andin series with the digital delay circuit.
 20. The method of claim 16further comprising routing the external clock signal to the delay lockedloop via a memory device input pad.
 21. The method of claim 16 furthercomprising holding the analog delay circuit at a second fixed delaybefore the digital delay circuit completes the coarse phase adjustment.